Newly downgraded Tropical Storm Isaac plodded its way across Louisiana on the seventh anniversary of Katrina Wednesday, causing massive power outages and forcing new evacuations.

Maximum sustained winds had slowed to 70 mph from 80 mph Tuesday evening, though rain continued to batter the region and was expected to dump 7 to 14 inches over much of Louisiana, southwest Mississippi and southwest Alabama.

A third of Louisiana households were without power Wednesday and emergency crews spent the morning rescuing residents of Plaquemines Parish, where the storm surge burst over 18 miles of levees, NBC News reported.

A state official says the levee in the hard-hit parish will be breached to relieve pressure on it. State Coastal Protection and Restoration Authority head Garret Graves says the levee won't be breached Wednesday because of weather conditions but it will happen as soon as they can get equipment to the site.

Isaac Storms The Gulf Coast

Rescue teams in Plaquemines parish, where the storm first made landfall, resuced between 60 and 75 people—everyone that needed to be rescued from floodwaters, Cpt. Lance T. Cagnolatti said. The National Guard and state police are still in the area, though divers had left the scene.

Plaquemines Parish has also ordered a mandatory evacuation for the west bank of the Mississippi River below Belle Chasse, worried about a storm surge. The order affects about 3,000 people in the area, including a nursing home with 112 residents.

Officials said the evacuation was ordered out of concern that more storm surge from Isaac would be pushed into the area and levees might be overtopped.

Joshua Brockhaus, an electrician who lives in the flooded area, helped rescue neighbors in his boat.

"I'm getting text messages from all over asking for help," he said. "I'm dropping my dogs off and I'm going back out there."

The hurricane's impact was a surprise for him.

"We didn't think it was going to be like that," he said. "The storm stayed over the top of us. For Katrina, we got 8 inches of water. Now we have 13 feet."

Alvin Sylve, a disabled former long-distance truck driver, was preparing to evacuate. He lived on street with rows of doube- and single-wide trailers in Jesuit Bend in Plaquemines Parish, an area outside the federal levee system where people were ordered to evacuate Wednesday as conditions worsened.

"We've never seen it this bad," he said. "The way this wind is shifting."

He was hunkering down at a friend's double-wide with leaks springing in the ceiling.

"This double-wide is shaking, even though it's anchored down. You see another piece came off the roof," he said, pointing to a flying piece of roof tile. "It's falling apart!"

Meanwhile in New Orleans, Mayor Mitch Landrieu issued a curfew for the city as Hurricane Isaac lashed the city on the seven-year anniversary of Hurricane Katrina's destructive arrival. The curfew was issued to prevent looting and to make it easier for utility crews to restore electricity. So far, there had been only sporadic arrests for looting.

Police cars had been patrolling the nearly empty streets since Isaac began bringing fierce winds and heavy rains to the city Tuesday night. The curfew was set to start Wednesday night and would last until further notice.

As of Wednesday afternoon more than 160,000 residents of Orleans Parish were without power, while more than 650,000 had lost power state-wide, according to NBC News. Airports in New Orleans remained closed Wednesday and Louis Armstrong New Orleans International Airport announced all commercial flights would be cancelled Thursday, NBC News reported.

In Vermilion Parish, Sheriff Mike Couvillon said a 36-year-old man had gone to help two friends move a vehicle from under a tree to prepare for Isaac on Tuesday evening, and fell to his death after climbing 18 feet up a tree. Deputies don't know why the man climbed the tree.

Rescuers in boats and trucks plucked a handful of people who became stranded by floodwaters in thinly populated areas of southeast Louisiana. Authorities feared many more could need help after a night of slashing rain and fierce winds that knocked out power to more than 600,000 households and businesses.

Although Isaac was much weaker than Katrina, which crippled the city in 2005, the threat of dangerous storm surges and flooding from heavy rain was expected to last all day and into the night as the immense comma-shaped storm crawled across Louisiana.

Army Corps spokeswoman Rachel Rodi said the city's bigger, stronger levees were withstanding the assault.

"The system is performing as intended, as we expected," she said. "We don't see any issues with the hurricane system at this point."

There were initial problems with pumps not working at the 17th Street Canal, the site of a breach on the day Katrina struck, but those pumps had been fixed, Rodi said.

In Plaquemines Parish, a fishing community south of New Orleans, about two dozen people who stayed behind despite evacuation orders needed to be rescued.

"I think a lot of people were caught with their pants down," said Jerry Larpenter, sheriff in nearby Terrebonne Parish. "This storm was never predicted right since it entered the Gulf. It was supposed to go to Florida, Panama City, Biloxi, New Orleans. We hope it loses its punch once it comes in all the way."

Water driven by the large and powerful storm flooded over an 18-mile stretch of one levee in Plaquemines Parish. The levee, one of many across the low-lying coastal zone, is not part of the new defenses constructed in New Orleans after Katrina.

After maintaining hurricane strength through the morning, Isaac weakened to a tropical storm Wednesday afternoon with 70 mph winds and was expected to continue losing strength. It came ashore at 7:45 p.m. EDT Tuesday with 80 mph winds near the mouth of the Mississippi River, driving a wall of water nearly 11 feet high inland and soaking a neck of land that stretches into the Gulf.

The storm stalled for several hours before resuming a slow trek inland, and forecasters said that was in keeping with the its erratic history. The slow motion over land means Isaac could be a major soaker, dumping up to 20 inches of rain in some areas. But every system is different.

Isaac's winds and sheets of rain whipped New Orleans, where forecasters said the city's skyscrapers could be subject to gusts up to 100 mph.

In the French Quarter near Bourbon Street, Jimmy Maiuri stepped outside his second-floor apartment to shoot video. Maiuri, who fled from Katrina at the last minute, stayed behind this time and had no regrets, though he was amazed at the storm's timing.

"It's definitely not one to take lightly, but it's not Katrina," he said. "No one is going to forget Aug. 29 forever, not here at least."

As hard wind and heavy rain pelted Melba Leggett-Barnes' home in the Lower 9th Ward, an area leveled during Katrina, she felt more secure than she did seven years ago.

"I have a hurricane house this time," said Barnes, who has been living in her newly rebuilt since 2008. She and her husband, Baxter Barnes, were among the first to get a home through Brad Pitt's Make It Right program.

Her yellow house with a large porch and iron trellis was taking a beating, but holding strong. "I don't have power, but I'm all right," said Barnes, 56, a cafeteria worker for the New Orleans school system.

In Mississippi, the main highway that runs along the Gulf, U.S. 90, was closed in sections by storm surge flooding. At one spot in Biloxi, a foot of water covered the highway for a couple of blocks, and it looked like more was coming in. High tide was likely to bring more water.

In Pass Christian, a Mississippi coastal community wiped out by hurricanes Camille and Katrina, Mayor Chipper McDermott was optimistic Isaac would not deal a heavy blow.

"It's not too bad, but the whole coast is going to be a mess," he said.

Tens of thousands of people had been told ahead of Isaac to leave low-lying areas of Mississippi and Louisiana, including 700 patients of Louisiana nursing homes. Mississippi shut down the state's 12 shorefront casinos.

The hurricane also canceled commemoration ceremonies Wednesday for Katrina's 1,800 dead in Louisiana and Mississippi.

The storm drew attention because of its timing __ coinciding with the Katrina anniversary and the first major speeches of the Republican National Convention in Tampa, Fla. Republican presidential candidate Mitt Romney was considering whether to visit the Gulf Coast after Isaac.

Isaac promised to test a New Orleans levee system bolstered by $14 billion in federal repairs and improvements after the catastrophic failures during Katrina. But in a city that has already weathered Hurricane Gustav in 2008.

Isaac also posed political challenges with echoes of those that followed Katrina, a reminder of how the storm became a symbol of government ineptitude.

President Barack Obama sought to demonstrate his ability to guide the nation through a natural disaster, and Republicans tried to reassure residents as they formally nominated former Massachusetts Gov. Mitt Romney as their presidential candidate.

"We are going to make sure to do every single thing we need to do to ensure that the folks down there are taken care of and have the support and love of the rest of this country," Obama said at the start of a campaign event in Virginia on Wednesday. "When things like this happen, there are no Democrats, and there are no Republicans. There are just Americans, and we stand by Americans in their hour of need."

There was already simmering political fallout from the storm. Republican Gov. Jindal, who canceled his trip to the convention in Tampa, said the Obama administration's disaster declaration fell short of the federal help he had requested. Jindal said he wanted a promise from the federal government to be reimbursed for storm preparation costs.

FEMA Administrator Craig Fugate said such requests would be addressed after the storm.